Sorbic acid is currently produced by decomposing, either with hydrochloric acid, alkali, or heat, a polyester prepared by reacting crotonaldehyde with ketene. The method using hydrochloric acid involves no isomer production, and yields a product of the best quality with the highest efficiency. However, all of the conventional methods require a separate purifying step since the tar produced as a by-product during the reaction remains as an impurity together with the polyester residue. Even the relatively advantageous method using hydrochloric acid as a decomposing agent is not an exception, and the crude sorbic acid obtained by cooling and filtering the decomposed polyester is not entirely free from tar. The need for removal of the residual hydrochloric acid is an additional problem.
Conventionally, the crude sorbic acid is purified by treatment with activated carbon or by recrystallization from water or a mixture of water and an organic solvent. However, either of these techniques suffers from considerable disadvantages in economic terms resulting from the adsorption of the sorbic acid and its retention in the filtrate. Furthermore, the commercial operation of these methods requires complicated equipment.
The crude sorbic acid may be purified by distillation. However, sorbic acid is structurally liable to heat, and as soon as it is melted, it starts to polymerize, this tendency being increased as the sorbic acid becomes less pure. It is therefore almost impossible to sufficiently purify the crude sorbic acid by commercial distillation. Co-distillation techniques that claim to have solved this problem and which are superior to the other methods have been disclosed in German Pat. No. 1,044,803 and Japanese Patent Publication No. 4091/69. However, even these techniques are not completely satisfactory, for several reasons, such as the tendency of the sorbic acid to form a resin. The biggest problem is that hydrochloric acid remaining in the crude sorbic acid that has been decomposed with hydrochloric acid distills off with the sorbic acid in the subsequent distillation step and is carried into the crystals of purified sorbic acid.